Another cluster includes six men aged between mid-30s and 70 who have been treated at the Norfolk and Norwich University Hospital.

And three people are being treated in a Shrewbury hospital for the disease - but are not thought to have contracted it from a common source.

Since the start of 2006 a total of 273 cases have been reported, compared to 205 up to the end of August in 2005.

Dr Carol Joseph, who is leading the HPA investigation, said: "It is usual to see an upsurge in cases of Legionnaires' disease at this time of year, however we would normally expect more cases to be associated with travel abroad and only a small proportion of these recent cases have travelled outside of the UK.

"The increase is made up of small clusters around the country.

"Although we currently don't have any evidence to link the individual cases in these small clusters our colleagues in local Health Protection Units are investigating them.

"We are also observing the picture nationally to ensure there is no information linking these small clusters."

Cause not known

Dr Joseph said it was difficult to pin down the reason for the rise in cases.

"However, as well as the seasonal rise at this time of the year, an increased awareness of the disease and greater use of rapid testing methods is probably contributing to more cases being detected across the country."

Legionnaires' disease, caused by a bacterium, is most often contracted by inhaling mist from water sources such as whirlpool baths, showers and cooling towers.

The most common cause is contaminated air conditioning systems.

Patients show flu-like symptoms in the early stages which can develop into fever and breathing difficulties.

It is fatal in 5-15% of cases, but cannot be passed from one person to another.

The disease can affect people of all ages but it mostly affects those over the age of 50 years and men more than women.

People should be advised to seek medical attention if they have a respiratory illness suggestive of Legionnaires' disease.